Astronaut-acquired Orbital Photographs as Digital Data for Remote Sensing:
Spatial Resolution
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Abstract:
Astronaut-acquired orbital photographs (astronaut photographs) are a useful
complement to images taken by orbiting satellites. They are in the public
domain, and have been particularly useful for scientists in developing
countries, as supplementary low-cloud data, and for studies requiring
large numbers of images. Depending on camera, lens and look angle, digitised
astronaut photographs can have pixel sizes representing areas on the Earth
as small as 10 m or less, although most photographs suitable for digital
remote sensing have pixel sizes between 30 and 60 m. Our objective is
to provide a practical reference for scientists in a variety of disciplines
who want to use astronaut photographs as remote sensing data. We detail
the characteristics of astronaut photography systems that influence spatial
resolution and summarise previous image acquisitions relative to these
elements. We present methods for estimating ground coverage under three
different levels of assumptions, to meet accuracy needs of different users.
Of the more than 375,000 photographs taken to date, at least half have
the potential to be used as a source of digital remote sensing data.
1. Introduction
1.1. Overview of NASA Astronaut Photography
Astronaut photography of Earth is produced
and archived by the National Aeronautics & Space Administration (NASA)
and provides an important record of the state of the Earth that has not
been used to its potential (e.g. Lulla et al. 1996). The practice was
the foundation for the development of other forms of orbital remote sensing
(Lowman 1999). Although the geometry is more complex than that of a vertical
aerial photograph, astronaut photographs still provide information that
can be interpreted by knowledgeable observers (Ring and Eyre 1983, Lowman
1985, Rasher and Weaver 1990, Drury 1993, Campbell 1996:121-156, Arnold
1997).
Official NASA campaigns of terrain, ocean,
and atmospheric photography were carried out during the Gemini missions
(Underwood 1967, Lowman and Tiedemann 1971), the Earth-orbiting Apollo
missions (Colwell 1971), the Apollo-Soyuz mission (El-Baz 1977, El-Baz
and Warner 1979), Skylab (NASA 1974, Wilmarth et al. 1977,), a few Shuttle
missions (e.g. the two Space Radar Laboratory missions of 1994, Jones
et al. 1996), and the Shuttle-Mir missions (Evans et al. 2000). Extensive
training in photography is available to members of all flight crews, during
their general training period and during intensive training for specific
missions (Jones et al. 1996). Most photographs have been taken by astronauts
on a time-available basis. Astronaut photographs are thus a subset of
the potential scenes, selected both by opportunity (orbital parameters,
lighting, and crew workloads and schedules) and by the training, experience,
and interest of the photographers.
As remote sensing and geographic information
systems have become more widely available tools, we have collaborated
with a number of scientists interested in using astronaut photography
for quantitative remote sensing applications. Digitised images from film
are suitable for geometric rectification and image enhancement followed
by classification and other remote sensing techniques (e.g. Lulla and
Helfert 1989, Mohler et al. 1989, Helfert et al. 1990, Lulla et al. 1991,
Eckardt et al. 2000, Robinson et al. 2000a, c, in press, Webb et al. in
press). Because the photographs are in the public domain, they provide
a low-cost alternative data source for cases where commercial imagery
cannot be acquired. Such cases often include studies in developing countries,
in areas that have not usually been targets for major satellites, needing
supplemental low-cloud data, requiring a time series, or requiring a large
number of images.
1.2. Extent of the Dataset
As of 30 September 1999, 378,461 frames
were included in the photograph database (Office of Earth Sciences 2000)
comprising 99 missions from Mercury 3 (21 July 1961) through STS-96 (27
may through 6 June 1999). We reviewed the database, removing photographs
that were deemed unsuitable for remote sensing analysis because they were
not Earth-looking (no entry for tilt angle), had no estimated focal length,
were over- or underexposed, or were taken at oblique tilt angles (further
discussion of these characteristics follows). The remaining 190,911 frames
(50.4% of the records present in the database) represent photographs that
are potentially suitable for use as remote sensing data. As the database
is always having new photography added, statistics can be updated on request
using the web link 'Summary of Database Contents' at the Gateway
to Astronaut Photography of Earth (Office of Earth Sciences 2000).
1.3. Objectives
The overall purpose of this paper is to
provide understanding of the properties of astronaut-acquired orbital
photographs to help scientists evaluate their applicability for digital
remote sensing. Previous general descriptions of astronaut photography
have been much less extensive and have tended to focus on numbers of photographs
taken and geographic coverage, and emphasised interpretative applications
(e.g. Helfert and Wood 1989, Lulla et al. 1993, 1994, 1996). By providing
a unique compilation of the background information necessary to extend
the use of astronaut photography beyond interpretation to rigorous analysis,
we hope to provide a resource for scientists who could use this data in
a variety of disciplines. In keeping with a potential interdisciplinary
audience, we have tried to provide sufficient background information for
scientists who do not have extensive training in remote sensing.
We focus on spatial resolution because it is one of the most important factors determining the suitability of an image for a remote sensing objective. In remote sensing literature, spatial resolution for aerial photography is often treated very differently than spatial resolution of multispectral scanning sensors. To provide sufficient background for a discussion of spatial resolution for a data source that shares properties with both aerial photography and satellite remote sensing, we first summarise the ways that spatial resolution is typically quantified for aerial photography and satellite remote sensors.
Given this background information, we (1) describe and illustrate factors that influence the spatial resolution of astronaut photographs, (2) show examples of estimating spatial resolution for a variety of photographs in a way that will enable users to make these calculations whether or not they have a background in photogrammetry, and (3) compare spatial resolution of digital data extracted from astronaut photographs to data obtained from other satellites.
